1. Field of the Invention
The present invention relates to a photoelectric smoke sensor installed in a hazardous place including a risk of ignition or explosion and having an explosion-proof structure.
2. Description of the Related Art
Smoke sensors have an advantage of being able to detect the occurrence of fire early. A photoelectric smoke sensor, which is a type of the smoke sensors, comprises a light emitting portion and a light receiving portion in a region where air enters and exits, and the region is shielded from external light by a plurality of shielding walls. When smoke accompanying fire enters, for example, a region inside the sensor, the light radiated from the light emitting portion is irregularly reflected by the smoke, the light receiving portion senses the irregularly reflected light, and the occurrence of fire is thereby detected.
The photoelectric smoke sensor comprises a circuit board electrically connected to the light emitting portion and the light receiving portion. In the circuit board, for example, an electric spark may be generated due to a defect or the like of an electronic component mounted on the circuit board or the temperature of the circuit board may abnormally increase due to deterioration of insulation resistance of the circuit board. If an electric spark or a high-temperature circuit board contacts combustible gas, the combustible gas is ignited and causes explosion.
For this reason, when a photoelectric smoke sensor is installed in a hazardous place having a risk of ignition or explosion, a photoelectric smoke sensor having an explosion-proof structure needs to be adopted.
Patent Literature 1 (JP 3938750 B) discloses a photoelectric smoke sensor employing an explosion-proof structure called an intrinsically safe explosion-proof type. In the intrinsically safe explosion-type photoelectric smoke sensor, the occurrence of an electric spark enough to ignite the combustible gas on the surface of the circuit board is prevented by limiting the magnitude of the current flowing through the circuit board.
Patent Literatures 2 (JP S58-129145 U), 3 (JP S63-008538 A), and 4 (JP S63-020050 U) disclose a photoelectric smoke sensor in which the circuit board is positioned remote from the light emitting portion and the light receiving portion so as not to cause explosion on the surface of the circuit board.
More specifically, the photoelectric smoke sensor of Patent Literature 2 comprises a smoke detecting portion comprising a light emitting element and a light receiving lens, and an electric circuit portion separated from the smoke detecting portion, and the light receiving element of the electric circuit portion is connected to the light receiving lens via an optical fiber.
In the photoelectric smoke sensor of Patent Literature 3, the light emitter, the direct light receiver and the scattered light receiver accommodated in a metal casing are coupled to a dark box into which the smoke flows by first to third optical fibers, and lenses are provided at ends of the first to third optical fibers positioned in the dark box, respectively.
In the photoelectric smoke sensor of Patent Literature 4, a labyrinth base on which smoke flows is separated from a light emitting element and a light receiving element, and the light emitting element and the light receiving element are connected via optical fibers, respectively.